The bacterium Rhizobium meliloti produces exopolysaccharides, including succinoglycan and EPSII, which mediate invasion of alfalfa root nodules during establishment of symbiotic nitrogen fixation. The wild type, or normal forms, of these polysaccharides includemodifications, such as acetyl, succinyl, and pyruvyl groups. We have used genetic analyses to identify genes involved in adding particular groups to succinoglycan and have determined that mutations in two of these genes cause symbiotic defects. We have also determined that nutrients in bacterial growth media can determine the extent to which succinoglycan is modified by particular groups. Our aim is to determine how exopolysaccharides mediate nodule invasion, by generating more structural variants of these exopolysaccharides and comparing their biological activities in terms of mediating nodule invasion by the bacterium.